Tone control system



Dec. 13, 1949 J, BOBB 2,491,155

TONE CONTROL S'IYSFIEM Filed Dec. 5l, 1943 2G .(26 w// Cz, C5.

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A INVENTOR. loyzefog rues Patented Dec. 13? 1949 t ce.

UNITED l sTAT 'r'oNE CONTROL sYs'rEM Lloyd J. Bobb,

assignments, to Philco Corporation,

Glenside, Pa.,v assignor, by mesne Philadelphia, Pa., a corporation of Pennsylvania Application December 31, 1943, Serial No. 516,557

claims. (C1. 17g-44) This invention relates to tone control systems the tone, or the bass and treble compensation,

may be adjusted in-conformity with the auditory characteristics of the human ear.

From one view point, an Aideal audio frequency amplifier should have a response which is linear and level over the whole audio frequency range. While this is a desired electrical obj ective, it does not take into account ther frequency response of the human -earnor, for that matter, the many variations introduced as the result of the inclusion in the circuits of devicesy having non-uniform frequency response characteristics such as loudspeakers and phonograph recording and'reproducingA elements. For example, in certain types of phonograph recordingequpment, it is essential to limit theivolume peaks to prevent'the driving of the'recording stylus -frovm one groovel into or across the next adjacent groove. This means that if the drums, or bass notes, predominate they must be recorded at a volume level materially less than their actual level. It followsthat when such a recording is reproduced it is highlydesirable togrestore kthe bass notes to theiroriginal volume in 'order that the tone reproduction may beof the desired high fidelity. v Y

In addition to the foregoing, it is well understood that if the volumerbe decreased, there `is a more rapid apparent loss of the frequencies at either end of thev audible frequency spectrum as compared with theintermediate frequencies; that is the ear, with decreasing volume, requires higher relative energy for the high and low notes than for those in between.y This means that the most satisfactory tone control should be capablejof operation correspondingly to increase the bass and treble response without materially disturbing the amplitude of the -tones in between. o

In carrying out the present invention in one form thereof, `a, tone'- control system is provided in which the range of frequencies vat each end of the audible spectrum may be increased in'amplitude by amounts which generally Yconform with the response of the human' ear.` l

The invention is characterized bythe provi.- sion of a novel tone compensating network adaptedfor insertion in an audio system and having series and shuntv branches. .A variable anda normally `fixed resistorare included in both the series and shuntbranches ofthe tone cornpenel satine-network'- ,The values V0f these: resistors adegree which will 2 bear a relation one to the 'other to provide a voltage division independent of simultaneous variation in the fsame direction of the variable resistors. This means that so far as the resistance network is concerned the-output voltage remains substantially constant regardless of the setting of the variable resistors, whereas the impedance of the tone compensating network may be changed over a relatively wide range to produce the desired tone control. Y

For a more detailed explanation ofthe invention, and; for. further objects andv advantages thereof, reference is to be had to the following de# scription taken in conjunction with 'the vaccom'- panying drawing','in which:

Fig. 1 is a wiring diagram of a typical embodiment of the invention; and Y :,Figs. 2 and 3 are graphs explanatory 'of the manner in which one feature of the system affects operation.

Referring to the drawing, the invention in one form comprises a lnetwork connected between the output circuit ofthe audio amplifier tube I5 and the output terminals I2 and I3. The tube I5 preferablyl has'a relativelyy low plate resistance. Since the portion of Fig. 1 including tube I5 is conventionahdetailed description of this portion isunneces'saryf' Intone embodiment of the-inventionthe plate resistance of thetriode t5 amounted to 7700 ohms while the resistor.20 hada value of around 50,000 ohmsxThe output from theA tube I5 is applied, by Way of a coupling capacitor 22, to the tone'compensating'rieti/vork. Inaccordance with the embodiment shown i'n thel drawing, this Vnetwork comprises aseries branch including a fixed resistor R1 and a variable resistor R2, and a shunt branch including a variable resistor R3, xed resistor R4 and acapacitor C1.' The' resistors R1 and R2 may be'shunted by capacitors C2 and C3, respectively,for purposes hereinafter described. The input signal lto the network, voltage Ep, maybe applied between the left hand terminal of resistor Rr andground, while the output voltageEo is obtained from terminals I2 and- I3 connected respectively to the junction of variable resistors Rz and Ra-and to ground. The variable resistors R2 and IRa are preferably mounted on the same shaft and inl any event the resistances thereof are varied simultaneously and in the s-ame direction. This means that when-R3 equals 0,- Rz equals 0; and When-R2 'isa maximum vR3 isa maximum.

The'xed resistors R1'- and Rimay have the same'- resistance values although-it is preferred that R1- be relatively high with respect to R4 andi with respect to the plate resistance of triode I and further, that R1 and R4 bear a predetermined ratio one to the other. The ratio of R2 with respect to Ra is then made the same as the ratio of R1 with respect to R4. In one modification of the invention the resistors R1 to R4, inclusive, respectively had resistance values o'fc-10'0;000..ohms, 1 megohm1'00,000 ohms and 10,000 ohms. In the absence'of the "condensers C1, C2 and Ca, the output voltage Eo across the terminals I2 and I3 is then determined Y by the ratio of the resistance (R1, rl'tz)v `inf-:the series branch to the'- resistance (R3, R4) in the shunt branch. Therefore, iffthe-'fresistance of R2 and R3 be varied in the^sameldirectiorr1the voltage division between the two branches will be constant and independent o'i' the .settingsotRz and R3. In other words the ratio R1-i- Rz Jiri-.R4

ywill always-be kthe same andwtl'izthel values used, by way of illustration, -thisaratio willzalwaysbe equalto 10.

The'foregoing neglects `theacapacitive` reactance of the Vcapacitor 1.C1.`:v The stated.ratio,ihowever, is closely approximatedfso longasthe'capacitive reactanee. or thefn'ipedance.r ofrithevcapicitor `C1 is small 'compared'` toV R3 and Thenapacity of C1 is, therefore, selected torfgivextheahovestated' condition above. vthe i range of', y say 800 to1000.cyc1es;perfsecond. .Below this range,:the empedance of thefcapacitonCi rises 1-rapidlywith decreasing frequency, and; produces :a corresponding increase in the :magnitude .of the output voltage En .for low frequencies '.only.` The capacitor C1 may be selected,..so"that-;at.;the.low.frequency endv of .thedesirediband and :with Rzland R3 adjusted Vvto theirminimum resistance'fvalues, Eo is not substantially less than the?.input-voltagel Ep which isapplied vto. the .tone compensating network.: Whenpthe resistance ofQRi tandnRa is reduced to zero, it will @be .understoodrthatLthe eect of the Variation of- '.impedance? of capacitor C1 with frequencyrwillfcause a:correspondingly greater: increase .in the output. vcltagez/Eufthanzin the case .when Rz .and .Ra iareismaximum. This follows since the impedance of the-:.capacitor.C1 at the lower rfrequenciesWill-i :then-liber lai-.gemas compared tothe'. sum of R3 "i Withthe. resistance values suggested Yabove', it follows that .the aratio-l of R1; plusl.; tor .the.im pedance' of Vthe .capacitor C1 can. belmade; to vary by a .factor of..11fby.adjustmentA ofsresistornRs. Thustthe rise in the-output;voltage'Enadue torise in impedance Loffthe ,capacitoraCr at .-flow'` frequencies may be controlledLWith a'apacity-'Ci of 0.03 Vmicrofarady .the.:.bassvrise-may i. be. around 1.25 decibels at 80 cyclesfwitliltz*fi-.Ra.azn'iaximuxn However when R24-.Rente a minimum;` ori-*equal to zero, thexbass rise at; 80 rcycles .willlbearound` decibels. `The' 'range' of'corrections :provided by an increase in the .bass-response `rfrom :11;25 decibels tofaround 15. decibels. .is 1 .adequate-.not only to compensate for a; frequencydistorting device included.'inthe. circuit, lbut it islralso adequate'` to 4emphasize :thexbassxnotes: or..to`nesto conformv Ywith the desire'siotfthose :wholike 'fthe low "frequency, end '-of' 'lthe fsciundr 1. spectrum emphasized.

' If Yit is desiredtdprovide 'even greater-bass emphasisythe condenser AC1A may be flshnted by an inductance of- 'such magnitude as to A'produce shuntresonance Vat the low frequencyend ofthe band.

4 1- The capacitors C2 and C3, respectively, shunting the resistor R1 and R1 serve to emphasize the notes at the upper, or high frequency, end of the audible spectrum. Without the capacitors 5 C2 and C3, the effect of the distributed capacitance between the leads, and between the grid 'and cathode of the amplifier stage connected to .the terminals lzfand I3, is to reduce. or: attenuate the ihigh'frequency sounds. This distributed l0 capacity, lumped together, is indicated by the capacitor C4. When the resistors R2 and R3 4vatea maximum the capacitor C1 is most effective in attentuating the high frequencies. The re- -rsponse may, for-example, be reduced as indi- 15catedby thel curve 25. However, by including the capacitors C2 and C3 the attenuation of the high frequencies is,.electively eliminated and a response as indicated by the curve 26 may be obtained.

With the resistors Rz and Ra at a minimum, `corresponding -to av maximum bass response, it will be understoodthat the reiect of thecapacitor C4 in Aattenuating the 'highen frequencies is a minimum. In other words, ltheimpedance of the f5 system looking back from vthe terminalsI I2 and I3 is a minimum. .-In' this case the capacitorCz emphasizes the higher frequencies It serves to increase Eo at vthe higher frequencies. In a typical embodiment'of the invention the' capacitor 30 Czhada capacitance of'400 micromicrofarads while the capacitor vC3 had a` capacity4v of :50 micromicrofarads. f Preferably the same ratio, around 10 t'o 1; is maintained vbetweenthecapacitors'Cz and C3 as betweenthe resistors Ri'and Rz,

so -that vwhen R2 is maximum the time constant of 4'the *circuit Altr-Cz is fthe same' as -Lthe time constant of the circuit Rz-Cz. i "With Rz and' Rrr-'adjustedV to 'their minimum values,and withoutfthe.I capacitors -Cz and .-Ca,

i0 the frequency responsewouldbeas indicated by the vcurve v2'! of I Fig. 3 but with =the 'capacitors the gain for' emphasisi'th'ereof is indicated -by the curve-28.

f With a-properly-'designed amplier,the over- '45 allresponse withrthe'tonefcontrol set for minimum compensation is indicated-by the -curve- 26 whichl itwillfbe observed: `islzllneanover the full range' of thef audible frequencyspectrum except for fa slightemphasis' ait' the-very -low frequencies.

5" However, the tone control set for maximum compensation; -they 'over-all responsey curve L' 2 8 shows vthe substantialfemphasis of 'thefsou'nds of low? frequency as well as the substantialvemphasis ofthev sounds at vthe `higher frequencies.

Sounds atthe intermediate frequencies are em# phasizedA to amuch Vlesser-degree.V vThis not-only makes it possiblevto'select a tone =control position -such that compensation of the :bass notes, needed because: of' -their suppression .during `re- '60 cording, is'eiectivew' but it-'alsoprovides *ideal compensationl for 'operation' of rthe system at low sound levels. The'compensationis adequate to produce esthetic -eifects :byoveremphasia --As-the volume isedecreased, the sounds towhic'h 05 the fear'iirst` fails toj-respond :are those atthe endsf of the sound frequency spectrum.'i That one reason-why in relatively inexpensive radio receivers, the quality is .poor -when'thevolume is low. Inl accordance with the-presentV invention; the tone .conti-oly on -such Y receivers may be -setjffor maximumfcompensatiori and the 4volume then i reduced" to a very great' f degree y i without materialrlossfo'fquality?l The-emphasis of highs and Ilows indicated by'thecurve ideally com- 1I pensates forethe lacklofresponse otithe eanllhe quality or fidelity of reproduction of music having an abundance of highs and lows remains the same even though the volume has been greatly reduced.

The 'present invention has the further advantage of low cost. The required circuit elements are relatively inexpensive and the testing and adjustment of the circuits during manufacture is simple.

I claim:

l. In a tone control system including an electric valve having a relatively low output impedance and an output circuit for said valve, the combination of a fixed resistor having a resistance relatively high as compared with the plate resistance of said valve, a variable resistor having a resistance approximately ten times that of said fixed resistor, means connecting said two resistors in series in said output circuit, capacitors connnected across said fixed and variable resistors and having capacities whose ratio is approximately to 1, a second fixed resistor and a second variable resistor in circuit with said first-named resistors and respectively having resistance values which differ in the order of 10 to l, a capacitor connected in series with said second iixed and said second variable resistor, an output circuit connected across said second xed resistor, said second variable resistor and said last-mentioned capacitor, and means for simultaneously varying the resistance of said variable resistors in the same direction such that when the resistance of one is a minimum the resistance of the other is a minimum thereby to control the effectiveness of said last-mentioned capacitor in producing emphasis of frequencies in the lower range of the audible frequency spectrum.

2. An audio frequency control network including a series branch having a pair of series-connected resistors therein and a capacitor in shunt with each resistor, a shunt branch having a pair of resistors and a capacitor serially connected therein, said shunt branch being connected to the output end of said series branch, and means for simultaneously varying one of said series branch resitors and one of said shunt branch resistors in the same direction.

3. A tone control network for the audio frequency range, said network having series and shunt branches, said series branch comprising a variable resistive impedance and a capacitive impedance connected in shunt thereacross, said shunt branch comprising a variable resistive impedance and a capacitive impedance connected in series therewith; means for deriving an output voltage across said shunt branch; and means for varying said variable resistive impedances in both branches simultaneously in the same direction, the values of said resistive impedances and the relative rates of variation thereof being such that the ratio of the total resistive impedance of said shunt branch to the total resistive impedance of said network remains substantially constant irrespective of the setting of the variable means, the ratio of the capacitive impedance of said shunt branch to the combined capacitive and resistive impedances of said network at a given frequency being variable and dependent upon the setting of said variable means, whereby said output voltage at both the low and high frequency ends of the audio frequency range varies appreciably in accordance with the setting of said variable means, said output voltage at the intermediate audio frequencies remaining relatively constant irrespective of the setting of said variable means.

4. A tone control network for the audio frequency range, said network having series and shunt branches each including resistive impedances adapted to be varied from a maximum to a minimum value, said minimum value being larger than zero, the ratio between said maximum values of the two branches being substantially equal to the ratio between said minimum values, said series branch having capacitive impedance in shunt with said resistive impedance, said shunt branch having capacitive impedance in series with said resistive impedance; means for deriving an output voltage across said shunt branch; and means for varying the resistive impedances of both branches simultaneously in the same direction at such relative rates that the ratio of the instantaneous resistive impedance of the series branch to that of the shunt branch is maintained substantially equal to said first-mentioned ratio, whereby said output voltage at both the low and Vhigh frequency ends of the audio frequency range varies appreciably in accordance `with the setting of said variable means, said output voltage at the intermediate frequencies remaining relatively constant irrespective of the setting of said variable means.

5. A tone control network as claimed in claim 4 characterized in that the output voltages at the low and high frequency ends of said audio frequency range are increased as said resistive impedances are varied toward said minimum values.

LLOYD J. BOBB.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 1,931,664 Lavoie Oct. 24, 1933 1,938,256 Jacobs Dec. 5, 1933 2,037,842 Wolff Apr. 21, 1936 2,069,853 Schade Feb. 9, 1937 2,225,337 Fyler Dec. 17, 1940 2,264,715 Rohr et al Dec. 2, 1941 

